As an apparatus installed in collective housing or a building for supplying water to each of water supply ends, there has been a water supply apparatus. FIG. 1 shows a typical example of such water supply apparatus. The water supply apparatus includes two pumps 1 combined with respective motors M for pressurizing and delivering water, and inverters (frequency converters) 2 for supplying electric power to the motors M for driving the respective pumps 1. The water supply apparatus includes a pressure tank 3 and a discharge-side pressure sensor 4 at the discharge side of the pumps 1, and flow switches (flow rate detecting means) 6 and check valves 7 for the respective pumps 1. A suction-side pipe 8 of the pumps 1 is connected to a water main 9. A suction-side pressure sensor 10 and a backflow prevention device 11 are provided in the suction-side pipe 8. Further, a bypass pipe 12 for supplying water only by the pressure of the water main 9 is provided between the suction-side pipe 8 and a discharge-side pipe 13 for the pumps 1. A check valve 14 is provided in the middle of the bypass pipe 12. A controller 15 for controlling the pumps 1 controls the rotational speeds of the pumps 1 and the number of operating pumps 1 according to the situation, based on signals from these sensors.
If the water supply apparatus is not a directly connected water supply apparatus whose suction-side pipe of the pump is connected to the water main, but is a receiving tank type water supply apparatus, then the suction-side pipe of the pump is connected to a water receiving tank, and a water level detector provided in the water receiving tank is connected to the controller. The receiving tank type water supply apparatus is free of the backflow prevention device, the suction-side pressure sensor, and the bypass pipe.
FIG. 2 shows a required head curve A representing the relationship between a usage flow rate and a pump head required for the usage flow rate, and a (standard) control head curve B established based on the required head curve A, as well as H-Q curves of the pump (rotational speeds N1, N2, N3 of the pump). In FIG. 2, the horizontal axis represents the flow rate Q, and the vertical axis represents the pump head (head) H.
The required head curve A is determined from the sum (H1+H2+H3) of the head H1 of, for example, the building (the height of the highest floor of the building), the pressure H2 required for the water supply instrument (pressure loss caused by the water supply instrument), and the piping loss H3 depending on the flow rate. In the illustrated example, the required head curve A is plotted as a curve smoothly interconnecting a head PB0 required when the usage flow rate is nil and a head PA0 required when the usage flow rate is of a final point Q0.
The required head curve A is determined from the relationship between an ideal pump head and a usage flow rate. For actual designs, it has widely been customary to establish the (standard) control head curve B which is higher than the required head curve A by a margin of, e.g., a dozen %, and to control the rotational speed of the pump based on the control head curve B. The (standard) control head curve B is plotted as a curve smoothly interconnecting a head (lowest required pressure) PB1 which is higher than the head PB0, by a margin of a dozen %, required when the usage flow rate is nil, and a head (highest required pressure) PA1 which is higher than the head PA0, by a margin of a dozen %, required when the usage flow rate is of the final point Q0.
The control head curve B is stored in a memory of the controller 15 of the water supply apparatus shown in FIG. 1 as a function of the head and the rotational speed. Based on the control head curve B, the controller 15 controls the rotational speed of the pump 1 so that when the usage flow rate is Q1, the intersection U3 between the flow rate Q1 and the control head curve B will be at the operating point (rotational speed N1) of the pump 1, as shown in FIG. 2, for example.
FIG. 3 shows an example of an operation cycle of the water supply apparatus shown in FIG. 2. In FIG. 3, the horizontal axis represents time, and the vertical axis represents the rotational frequency of the pump. The rotational frequency of the pump 1 is controlled in a variable speed manner by the inverter 2.
As shown in FIG. 3, when the pump 1 stops its driving (time: t1), and then a discharge pressure DP measured by the discharge-side pressure sensor 4 becomes lower than a set pressure (setting pressure) SP (DP<SP), it is judged that water is used, and the pump 1 starts to rotate (time: t2) and supplies water. During supply of water, a PI calculation is performed by using the set pressure SP and the current discharge pressure DP measured by the discharge-side pressure sensor 4, and variable speed control of the pump 1 is performed by reflecting the PI calculation results in the rotational frequency of the pump 1.
Thereafter, during supply of water by the pump 1, when the discharge pressure DP becomes sufficiently high and the flow switch 6 is closed and thus a reduction of an amount of used water is detected (time: t3), the pump 1 starts a pressurizing operation for accumulating a pressure in the pressure tank 3 and then performs a small flow rate stopping operation that enables to use water in the pressure tank 3 at the subsequent small flow-rate usage, and then the pump 1 stops its driving (time: t4). The time to detect “closing” of the flow switch 6 is one second, for example.
Then, when the discharge pressure DP measured by the discharge-side pressure sensor 4 becomes lower than the set pressure SP (DP<SP), it is judged that water is used, and the pump 1 starts to rotate again (time: t5) and supplies water.
Here, the time (t1−t2) during which the pump 1 stops its driving is referred to as “pump stopping time”. The time (t2−t3) from when the pump 1 starts its driving till when “closing” of the flow switch 6 is detected is referred to as “pump immediately-before driving time”. The same shall apply hereinafter. The time (t1−t4) from when the pump 1 stops its driving and then the pump 1 starts its driving to supply water till when the pump 1 stops again corresponds to one cycle of the operation of the water supply apparatus. The water supply time, the pump immediately-before driving time, and the pump stopping time vary depending on the frequency of use of water and the rotational frequency of the pump 1 varies depending on the amount of used water of water.
Specifically, in a time zone when water is continuously used, “the pump immediately-before driving time” to drive the pump continuously is prolonged. In a time zone when water is not used much, “the pump stopping time” to stop the driving of the pump is prolonged. Further, if water is used in large quantities, the rotational speed of the pump is increased, and if only a small quantity of water is used, the rotational speed of the pump is lowered.
As described above, the (standard) control head curve B which is higher than the required head curve A by a margin of a dozen % is set, and the rotational speed of the pump is controlled based on the control head curve B. Therefore, for example, in the case where the water pipe is corroded, causing a greater piping loss than the initially designed piping loss, the water supply apparatus is prevented from failing to exercise the required performance in use and is able to meet the demand for an increase in the flow rate that the user may want to achieve for some reason.
There has been proposed a method of inputting a flow rate determined from the pipe resistance and the pump performance curve and automatically controlling the rotational speed of the pump in order to achieve a target flow rate (see Patent document 1). According to the proposed method, when the flow rate is initially measured, if the flow rate is high, then the rotational speed of the pump is automatically lowered. If the flow rate is still high regardless of the reduction in the rotational speed of the pump, then the rotational speed of the pump is further automatically lowered so as to meet the flow rate. In this manner, the rotational speed of the pump is automatically adjusted sequentially until the target flow rate is reached.